resvg silently skips emoji when raster-images feature is turned off

[si14]

Reproduction repo: https://github.com/si14/resvg-emoji-bug

With raster-images enabled:

Without:

That was quite fun to debug 😅

[RazrFalcon]

How about enabling logs? It would print "Images decoding was disabled by a build feature.".

[si14]

@RazrFalcon at the very least the requirement is not reflected in the documentation or the log message itself. Emoji fonts might secretly be images and thus require raster-images, but it's not trivially evident.

[RazrFalcon]

Yes, we should add logging to examples and update docs.

Emoji fonts might secretly be images

They are PNGs in 99% of the cases.